Synthesis of a phosphorus-containing hybrid porphyrin

Article abstract of DOI:10.1021/ol0622763

(Diagram presented) A phosphorus-containing hybrid porphyrin was successfully prepared via the BF₃-promoted dehydrative condensation between σ⁴-phosphatripyrrane and 2,5-bis[hydroxy(phenyl)methyl] thiophene. The NMR and UV-vis absorp

Full text of DOI:10.1021/ol0622763

ORGANIC
LETTERS
2006
Vol. 8, No. 25
5713-5716
Synthesis of a Phosphorus-Containing
Hybrid Porphyrin
Yoshihiro Matano,*,† Takashi Nakabuchi,† Tooru Miyajima,† Hiroshi Imahori,† and
Haruyuki Nakano‡
Department of Molecular Engineering, Graduate School of Engineering, Kyoto
UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and Department of Chemistry,
Graduate School of Sciences, Kyushu UniVersity, Fukuoka 812-8581, Japan
matano@scl.kyoto-u.ac.jp
Received September 15, 2006
ABSTRACT
A
phosphorus-containing hybrid porphyrin was successfully prepared via the BF3-promoted dehydrative condensation between σ4
phosphatripyrrane and 2,5-bis[hydroxy(phenyl)methyl]thiophene. The NMR and UV vis absorption spectra, electrochemical measurements,
3-P,N2,S-hybrid porphyrin exhibits high aromaticity as an 18
-electron system in terms of both geometric
-
−
and DFT calculations revealed that the
and magnetic criteria.
σ
π
Porphyrins are one of the most widely studied aromatic
macrocycles because of their important roles in biological,
materials, and coordination chemistry. Chemical modification
of the core of porphyrins is a highly promising approach to
developing new classes of dyes and catalysts, as the
electronic structures and coordination environments of the
porphyrin ring can be changed dramatically.1 In this respect,
much attention has been paid to the chemistry of core-
modified porphyrins, such as carbaporphyrins,2 N-confused
porphyrins,3 chalcogen-containing porphyrins,4 and pyripor-
phyrins.5
Incorporating a phosphorus atom at the core of the
porphyrin ring, namely, replacing a pyrrole ring with
phosphole, seems a quite promising strategy for altering
significantly the optical, electrochemical, and coordinating
properties of the porphyrin macrocycle. In 2003, Delaere and
Nguyen predicted the ground state electronic structures of
unsubstituted monophospha- and diphosphaporphyrins based
on density functional theory (DFT) calculations and con-
cluded that these porphyrins would exhibit reasonable
aromaticity.6 Experimentally, however, phosphorus-contain-
ing porphyrins have long been untouched.7 Recently, we
† Kyoto University.
(2) For example, see: (a) Lash, T. D. Angew. Chem., Int. Ed. Engl. 1995,
34, 2533. (b) Berlin, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1820. (c)
Liu, D.; Ferrence, G. M.; Lash, T. D. J. Org. Chem. 2004, 69, 6079. (d)
Stepien, M.; Latos-Graz˘yn´ski, L.; Szterenberg, L.; Panek, J.; Latajka, Z. J.
Am. Chem. Soc. 2004, 126, 4566. (e) Sprutta, N.; Swiderska, M.; Latos-
Graz˘yn´ski, L. J. Am. Chem. Soc. 2005, 127, 13108.
(3) For example, see: (a) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem.
Soc. 1994, 116, 767. (b) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.;
Rachlewicz, K.; Glowiak, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779.
(c) Furuta, H.; Maeda, H.; Osuka, A. J. Am. Chem. Soc. 2000, 122, 803.
(d) Rachlewicz, K.; Wang, S.-L.; Ko, J.-L.; Hung, C.-H.; Latos-Graz˘yn´ski,
L. J. Am. Chem. Soc. 2004, 126, 4420.
‡ Kyushu University.
(1) (a) Latos-Graz˘yn´ski, L. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000. Vol.
2, Chapter 14. (b) Chandrashekar, T. K.; Venkatraman, S. Acc. Chem. Res.
2003, 36, 676. (c) Sessler, J. L.; Seidel, D. Angew. Chem., Int. Ed. 2003,
42, 5134. (d) Furuta, H.; Maeda, H.; Osuka, A. Chem. Commun. 2002,
1795. (e) Srinivasan, A.; Furuta, H. Acc. Chem. Res. 2005, 38, 10. (f)
Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Coord. Chem. ReV. 2005, 249, 2510
(g) Stepien´, M.; Latos-Graz˘yn´ski, L. Acc. Chem. Res. 2005, 38, 88. (h)
Gupta, I.; Ravikanth, M. Coord. Chem. ReV. 2006, 250, 468 and references
therein.
10.1021/ol0622763 CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/10/2006

established a convenient method for the preparation of 2,5-
difunctionalized phospholes,8 which have proven to be
reliable starting materials for phosphorus-containing hybrid
calixpyrroles9 and calixphyrins.10 Here, we report the first
example of a phosphorus-containing hybrid porphyrin as a
new family of core-modified porphyrins. Both experimental
and theoretical results clearly demonstrate that the σ3-P,N2,S-
hybrid porphyrin possesses high aromaticity with a relatively
narrow HOMO-LUMO gap.
Scheme 1 illustrates the synthesis of σ3-P,N2,S-hybrid
porphyrin 6. Treatment of 2,5-bis(hydroxymethyl)-1-phenyl-
Figure 1. σ3-P,N2,S-Hybrid calixphyrin 7, porphyrins 8a,b, 21-
thiaporphyrins 9a,b, and σ3-P,N2,S-hybrid porphyrin 10.
Scheme 1. Synthesis of σ3-P,N2,S-Hybrid Porphyrin 6
dicyanobenzoquinone (DDQ) to afford the target compound
6 in 14% yield (from 4) as an air- and heat-stable reddish
purple solid.
The structure of σ3-P,N2,S-hybrid porphyrin 6 was fully
characterized by mass spectrometry and NMR spectroscopy.
In the HR-FAB-MS spectrum, the parent ion peak (M+) was
observed at m/z 612.1788 (calcd for C41H29N2PS, 612.1789).
In the 1H NMR spectrum, the meso, pyrrole-â, and thiophene-â
protons of 6 appeared at δ 10.44 (d, 3JP-H ) 18.8 Hz), 8.67
and 8.93, and 9.22, respectively, whereas the P-phenyl ortho-,
meta-, and para-protons appeared at δ 2.29, 5.27, and 5.66,
respectively (Figure 2). The significant downfield and upfield
1-thiophosphole (1)8 with excess pyrrole in the presence of
BF3‚OEt2 afforded σ4-phosphatripyrrane (2) in 14% yield.
The BF3-promoted dehydrative condensation of 2 with 2,5-
bis[hydroxy(phenyl)methyl]thiophene (3)11 in CH2Cl2 gave
σ4-P,N2,S-porphyrinogen (4) in 36% yield as a mixture of
diastereomers.12 Desulfurization of 4 with excess P(NMe2)3
in refluxing toluene yielded σ3-P,N2,S-porphyrinogen (5),
which was subsequently oxidized by 2,3-dichloro-5,6-
(4) For example, see: (a) Broadhurst, M. J.; Grigg, R.; Johnson, A. W.
J. Chem. Soc. C 1971, 3681. (b) Ulman, A.; Manassen, J. J. Am. Chem.
Soc. 1975, 97, 6540. (c) Latos-Graz˘yn´ski, L.; Lisowski, J.; Olmstead, M.
M.; Balch, A. L. J. Am. Chem. Soc. 1987, 109, 4428. (d) Sessler, J. L.;
Cyr, M.; Burrell, A. K. Tetrahedron 1992, 48, 9661. (e) Lee, C.-H.; Kim,
H.-J. Tetrahedron Lett. 1997, 38, 3935. (f) Narayanan, S. J.; Sridevi, B.;
Chandrashekar, T. K.; Ashwani, V.; Roy, R. J. Am. Chem. Soc. 1999, 121,
9053. (g) Abe, M.; Hilmey, D. G.; Stilts, C. E.; Sukumaran, D. K.; Detty,
M. R. Organometallics 2002, 21, 2986.
(5) (a) Mysliborski, R.; Latos-Graz˘yn´ski, L.; Szterenberg, L. Eur. J. Org.
Chem. 2006, 3064. (b) Sim, E.-K.; Jeong, S.-D.; Yoon, D.-W.; Hong, S.-
J.; Kang, Y.; Lee, C.-H. Org. Lett. 2006, 8, 3355.
(6) Delaere, D.; Nguyen, M. T. Chem. Phys. Lett. 2003, 376, 329.
(7) Quite recently, Mathey reported the synthesis of a phosphaporphyrin
analog that contains a σ4-phosphorus atom at the peripheral position. Mathey,
F. Abstract for the JSPS Second Core-to-Core Symposium on Main Group
Element Chemistry, Tokyo, 2006, p. 27.
(8) Matano, Y.; Miyajima, T.; Nakabuchi, T.; Matsutani, Y.; Imahori,
H. J. Org. Chem. 2006, 71, 5792.
Figure 2. 1H NMR spectrum of 6 in CDCl3 at 25 °C. Asterisks
(/) indicate the residual solvent peaks.
(9) Matano, Y.; Nakabuchi, T.; Miyajima, T.; Imahori, H. Organo-
metallics 2006, 25, 3105.
(10) Matano, Y.; Miyajima, T.; Nakabuchi, T.; Imahori, H.; Ochi, N.;
Sakaki, S. J. Am. Chem. Soc. 2006, 128, 11760.
appearances of these peaks are indicative of a ring current
effect derived from the porphyrin π-circuit. The 31P peak of
6 (δ 18.6) was also very shielded as compared to that (δ
26.7) of σ3-P,N2,S-calixphyrin 710 (Figure 1) and those (δ
32.0-36.0) of 2,5-disubstituted 1-phenylphospholes bearing
(11) (a) Chadwick, D. J.; Willbe, C. J. Chem. Soc., Perkin Trans. 1 1977,
887. (b) Jang, Y.-S.; Kim, H.-J.; Lee, P.-H.; Lee, C.-H. Tetrahedron Lett.
2000, 41, 2919.
(12) The 1H and 31P NMR spectra of 4 and 5 indicated that three
diastereomers were present. For details, see Supporting Information.
5714
Org. Lett., Vol. 8, No. 25, 2006

the same 3,4-C3 bridge.8,13 Thus, the hybrid porphyrin 6 is
likely to possess high aromatic character. It should be noted
here that the upfield shift (∆δ/ppm) of the P-phenyl protons
of 6 relative to the corresponding protons of 7 increases in
the order para (1.61) < meta (2.01) < ortho (5.27). This
implies that the P-phenyl group in 6 stands above the
porphyrin π-plane like the N-methyl group in N-methyl-
porphyrins.14 As a consequence, the ortho protons of the
meso-phenyl groups, which would be oriented at a slant
against the porphyrin plane, were differentiated at 25 °C.15
In the UV-vis absorption spectrum of 6 in CH2Cl2, the Soret
band appeared at λmax 440 nm (Figure 3a), which was red-
DFT calculations to reveal the optimized structures of σ3-
P,N2,S-hybrid 10, parent porphyrin 8b, and 21-thiaporphyrin
9b as models for 6, 8a, and 9a, respectively.16 As shown in
Figure S5 in Supporting Information, the porphyrin planes
of 8b and 9b are completely planar, whereas that of 10 is
nonplanar. Thus, replacement of the NH unit of thiaporphyrin
with the PPh unit (from 9b to 10) induces a distortion of the
porphyrin plane, where all the heterole subunits are slightly
tilted to make a dish-like macrocyclic platform. Such a
deviation is presumably caused by electrostatic repulsion
between the phosphorus and sulfur atoms in 10.17
The phosphorus center in 10 adopts a trigonal pyramidal
geometry (ΣC-P-C ) 299.5°), which prevents effective
interaction between the phosphorus lone pair and the
porphyrin 18π-circuit. Similar geometries at the phosphorus
center (ΣC-P-X ) 292.4-296.1°; X ) C, H) were reported
for the 21-phospha- and 21,23-diphosphaporphyrins bearing
P-H bonds,6 suggesting that the phosphorus lone pair in
σ,3λ3-P-containing porphyrins essentially has an s character
irrespective of the P substituents. The geometry of the
P-phenyl group in 10 explains the 1H NMR observation for
6 as well.
With the optimized structures in hand, we evaluated the
aromaticity of 10 by comparing it with those of 8b and 9b.
Despite the nonplanarity, the P,N2,S-hybrid 10 keeps high
aromaticity in terms of a geometric criterion. The mean
HOMA18 values for all 28 bonds of the porphyrin ring of
8b, 9b, and 10 were calculated to be 0.680, 0.645, and 0.550,
respectively (Table 1). The rather low value of 10 as
Table 1. HOMA and NICS Values for Model Compounds
porphyrin
(X, Y)
8b
(NH, NH)
9b
(NH, S)
10
(PPh, S)
Figure 3. (a) UV-vis absorption spectra of 6 in CH2Cl2. (b) Cyclic
voltammogram of 6; 0.1 M nBu4NPF6, Ag+/Ag (0.01 M AgNO3).
Scan rate 20 mV s-1
.
HOMA
totala
18πb
0.680
0.853
0.645
0.887
0.550
0.879
shifted as compared to those of 5,10,15,20-tetraphenylpor-
phyrin (TPP) 8a (λmax 411 nm) and 21-thia-5,10,15,20-
tetraphenylporphyrin 9a (λmax 425 nm). The Q-type transitions
of 6 were also observed at longer wavelengths, λmax 492,
518, 547, 647, and 718 nm. In contrast to 8a, σ3-P,N2,S-
hybrid 6 was not fluorescent.
NICS (ppm)
-16.5
-15.3
-4.49
R
â
γ
δ
-16.2
-15.9
-2.54
-19.0
-15.6
-16.7
-1.96
-19.8
a Calculated using all 28 bonds of the porphyrin ring. b Calculated using
18 bonds of the [18]annulene circuit.
All attempts to grow single crystals of 6 suitable for X-ray
diffraction analysis were unsuccessful. Thus, we carried out
compared to those of 8b and 9b may be ascribed to distortion
of the phosphole subunit.19 On the other hand, the HOMA
values for the 18π-circuit (18 bonds consisting of C and N
atoms) of 8b, 9b, and 10 are 0.853, 0.877, and 0.879,
respectively, implying high aromaticity of 10 as a bridged
annulene derivative. To evaluate the degree of aromaticity
(13) Hay, C.; Hissler, M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet,
L.; Nyula´szi, L.; Re´au, R. Chem. Eur. J. 2001, 7, 4222.
(14) It is known that the N-methyl protons in N-methylporphyrins are
significantly shielded as a result of the ring current effect. For example,
see: (a) Lavallee, D. K.; Gebala, A. E. Inorg. Chem. 1974, 13, 2004. (b)
Lavallee, D. K.; Anderson, O. P. J. Am. Chem. Soc. 1982, 104, 4707.
(15) These ortho protons (indicated as j in Figure 2) coalesced at 60 °C
in Cl2CDCDCl2.
Org. Lett., Vol. 8, No. 25, 2006
5715

of 10 on the basis of a magnetic criterion, we also calculated
NICS20 values at the porphyrin ring and heterole ring centers
(Table 1). The NICS values at the center of the core atoms
of 8b, 9b, and 10 are -16.5, -16.2, and -15.6 ppm,
respectively,21 indicating that the aromaticity of the P,N2,S-
hybrid 10 is slightly weaker than those of 8b and 9b.
Interestingly, the NICS value at the phosphole ring center
(â) in 10 (-16.7 ppm) is much higher than that of the parent
1-phenylphosphole (-4.31 ppm).22 This reflects that the
cyclic 1,3-diene unit of the phosphole ring is included in an
18π-electron system. Namely, the considerable ring current
effect derived from the 18π-circuit was detected at the â
position. By contrast, the NICS value at the γ position in 10
(-1.96 ppm) is much lower than the respective value of
pyrrole, reflecting the strong 2-azafulvene character of the
N-heterole rings.
To gain insight into the HOMO and LUMO energies of
the P,N2,S-hybrid porphyrin 6, redox potentials were mea-
sured by cyclic voltammetry (CV) and/or differential pulse
voltammetry (DPV). As shown in Figure 3b, the electro-
chemical oxidation process of 6 is irreversible, and the first
oxidation potential of 6 was determined by the DPV
measurement to be +0.45 V (vs Fc/Fc+),23 which is more
cathodic than the reported values for 8a (E1/2 +0.58 V) and
9a (E1/2 +0.62 V). On the other hand, electrochemical
reduction of 6 occurred quasireversibly, and the first and
second reduction potentials (E1/2) were determined as -1.36
and -1.56 V (vs Fc/Fc+), respectively, which are more
anodic than those of 8a (-1.73 and -2.06 V) and 9a (-1.55
and -1.82 V). These data suggest that modification of the
core of the porphyrin from an NH to a PPh unit basically
narrows the HOMO-LUMO energy gap.24
In summary, we successfully prepared a phosphorus-
containing core-modified porphyrin for the first time. We
found that the σ3-P,N2,S-hybrid porphyrin is composed of a
bridged [18]annulene π-system and displays high aromaticity
in terms of both geometric and magnetic criteria. This class
of compounds is expected to behave as macrocyclic, multi-
dentate phosphorus ligands with characteristic optical proper-
ties. In this context, the coordination chemistry of phosphorus-
containing porphyrins is worthy of further study.
(16) The structures were optimized by the DFT method at the level of
B3LYP/6-311G(d,p). In the optimized structure of 10, the P-phenyl ring
is vertical to the P-S axis. However, the difference in energy between the
optimized structure and the structure in which the P-phenyl ring is rotated
by 90° (i.e., the P-phenyl ring is parallel to the P-S axis) is very small
(1.87 kcal mol-1). Thus, the binding for the P-phenyl ring rotation is
considered to be very weak.
(17) It has been reported that substitution of an NH unit of 8b by
a PH unit does not distort the carbon skeleton of the porphyrin ring. See
ref 6.
(18) HOMA: Harmonic Oscillator Model of Aromaticity. (a) Kruszewski,
J.; Krygowski, T. M. Tetrahedron Lett. 1972, 3839. (b) Krygowski, T. M.
J. Chem. Inf. Comput. Sci. 1993, 33, 70. (c) Krygowski, T. M.; Ciesielski,
A.; Bird, C. W.; Kotschy, A. J. Chem. Inf. Comput. Sci. 1995, 35, 203. (d)
Cyran˜ski, M. K.; Krygowski, T. M.; Wisiorowski, M.; van Eikema Hommes,
N. J. R.; Schleyer, P. von R. Angew. Chem., Int. Ed. 1998, 37, 177. In this
paper, the HOMA value of 8b was reported to be 0.666 at the level of
B3LYP/6-31G(d).
Acknowledgment. This work was partially supported by
Grants-in-Aid (no. 17350018 and 21st Century COE on
Kyoto University Alliance for Chemistry) from the Ministry
of Education, Culture, Sports, Science and Technology of
Japan.
(19) In the phosphole ring of 10, the phosphorus atom is slightly deviated
from the 1,3-diene unit with a dihedral angle of 2.4°.
Supporting Information Available: Experimental de-
tails, 1H NMR spectra, and DFT computational results. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(20) NICS: Nuclear Independent Chemical Shift. The NICS values were
calculated at the level of GIAO-RHF/6-31+G(d) at the optimized geom-
etries. For details, see Supporting Information.
(21) For the NICS values of porphyrins, see ref 18d. See, also: Furuta,
H.; Maeda, H.; Osuka, A. J. Org. Chem. 2001, 66, 8563.
(22) At the same level of calculations, the NICS values of pyrrole,
thiophene, and 1-phenylphosphole were determined as -15.0, -13.2, and
-4.31, respectively. For the reported NICS values of these heteroles, see:
(a) Dransfeld, A.; Nyulaszi, L.; Schleyer, P. von R. Inorg. Chem. 1998,
37, 4413. (b) Delaere, D.; Dransfeld, A.; Nguyen, M. T.; Vanquickenborne,
L. G. J. Org. Chem. 2000, 65, 2631. (c) Cyranski, M. K.; Krygowski, T.
M.; Katritzky, A. R.; Schleyer, P. von R. J. Org. Chem. 2002, 67, 1333.
OL0622763
(23) The second oxidation potential of 6 was determined to be 0.97 V
(vs Fc/Fc+) by the DPV measurement.
(24) The HOMO-LUMO energy gaps (eV) of 8b, 9b, and 10 were
computed by the DFT method to be 2.94, 2.83, and 2.59, respectively.
5716
Org. Lett., Vol. 8, No. 25, 2006